Sorry if my response sounds harsh. What does "having an actual session" mean to you? If it means bondage and intense tickling where a safeword would usually be used, then I think this is something you should be up-front about pretty quickly with any gal. If she is a soon to be fiance, then I'm gona assume she has been your gf for a yr or 2. I would tell any gal within the 1st 1 or 2 months.
It is possible that she will not want to have a session. It is possible that you 2 could have a session and she doesn't like it. If you are here on tmf then you probably need tickling in your life from your partner.
So my advice is to tell her everything asap. Be 100% honest about what you would like her to give you. A lee is giving a lot in the average tickling session, trust, torture, their body, etc. You gota get this question nailed down asap. Definitely do it before marrying/proposing to her.
Of course, if you don't care about tickling that much, and a session would just be you poking her some then disregard what I said above lol.
If you are 100% honest with her and yall don't work out, you still did the right thing. I wish the best for you both 🙂